I don't like the Dekoni attenuator rings. Yes, they do raise the sub-bass output, but they also negatively effect the mids and highs some. And of all the pads I've tried I've only found one that really works for me, and it hasn't been made in many years. The following is what has been the magic-3 combo on the TH-X00 PH's for me.
1. Lawton Tune-up kit - driver damping only! At recent audio meet another person also had Lawton Tune-up kit (full) on his Mahoganies and with my pad setup swapped to the Magonies it was a noticeably difference to driver only application, bass impact reduced and overall more recessed sounding along with too restrained mids and highs. The Lawton kit (driver damping only) really improves the clarity and detail greatly...probably the largest impact on improving sound quality of a TH-X00. Gets rid of the stock congested/compressed sound quality and really brings out details you have never heard before along with very noticeable improvement in instrument seperation. #1 mod that should be done above anything else.
2. J-Money sheepskin earpads....somebody REALLY needs to make a clone of these pads. Mine are 8 years old that I got for my Denon AH-D2000's...those pads are the key to getting clear, live open headphone sound with rediculous closed headphone bass that does not roll into the mids. The mids and highs are clearer and more detailed as well.
3. Custom attenuators that go only in the back portion of the earpad, inside the earcup area. These can be made from a variety of thin stiff materials (manila folder/thin plastic/etc...) and shape experimented and played around with. Main thing is you don't want a round 'donut' with material in the front section of the earpad. The Dekoni donut attenuators, in comparison to the custom rear only attenuators, sound bloated and mess with the mid and high detail/clarity.
Picture of custom attenuator (which slips inside the ear opening of the earpad):